1. Field of the Invention
The present invention relates to circuitry for generating power-on reset signals for transmission to integrated circuit systems. In particular, the present invention relates to a power-on reset circuit that operates to hold off power-on reset until the high-potential power supply rail reaches a defined potential. More particularly, the present invention relates to power-on reset circuitry that operates as required when coupled to circuitry powered by supplies of differing potentials. The present invention is a power-on reset circuit used in association with logic level translation systems.
2. Description of the Prior Art
Power-on reset circuits are designed to transmit to semiconductor-based systems signals to enable operation of such systems only when a common high-potential power rail reaches a certain minimum potential. These power-on reset circuits may be used to enable "hot" or live insertion of systems or subsystems such as circuit boards that are initially unpowered into extended coupled circuitry that is powered. The power-on reset circuitry is supposed to protect the unpowered system or subsystem from significant initial potential variations that may cause damage or unexpected operational anomalies in the circuitry that is inserted into the active system. In effect, the power-on reset circuit is designed to hold off enabling activation of the subsystem until the power rail potential is suitable for activation of that particular subsystem.
A voltage level converter or translator buffer is used to adjust the logic high and logic low voltage levels associated with a single input signal, or a pair of input signals, coming into the buffer to high and low voltage levels compatible with downstream circuitry. The translator buffer must transfer these electrical signals at desired amplitude and rate and, preferably, using as little power as possible. The signal transfer occurs between active devices that are either on the same semiconductor-based chip or on different chips. The devices may be located proximate to one another, or they may be some distance from one another. One example of a proximate device interface requiring one or more bus connections is the coupling of one printed circuit board to another within a computing system, such as through a backplane bus. An example of a remote device interface requiring one or more bus connections is the coupling of one computing system to another.
It is well known that in digital systems the signals moving between devices are categorized as either logic level high (or "1" or "ON") and logic level low (or "0" or "OFF"). The particular signal potential that defines whether a logic high or a logic low is being transmitted is dependent upon the semiconductor components that form the circuitry associated with that transmission. The most common circuit configurations used to produce digital signals include, among others, CMOS, Transistor-Transistor Logic (TTL), and Emitter Coupled Logic (ECL). Each of these logic configurations operates differently as a function of the "swing" between what constitutes a logic high signal and what constitutes a logic low signal.
For CMOS logic systems for example, which is based primarily on the use of slower, less-power-consuming MOS transistors, a logic low signal is generally developed in the range of 0.6 volts (V) above a low-potential power rail GND, which may be at 0.0 V. A logic high signal is generally developed in the range of Vcc to Vcc-0.6 V, where Vcc may vary between 4.5 V and 5.5 V for a nominal 5-volt supply, or between 3.0 V and 3.6 V for a nominal 3.3-volt supply. For the 3.3-volt supply then, the differential swing between low and high must be at least 2.4 volts in order to ensure that a desired shift between a logic low and a logic high will occur. More recently, nominal 2-volt supplies are being used to power eversmaller devices. TTL and ECL logic configurations, on the other hand, are based primarily on the use of faster, high-power-consuming bipolar transistors. The differential swing for a shift between a logic low and a logic high is significantly less than it is for CMOS operation--it may as low as 0.4 volt. In TTL circuitry for example, which is Vcc dependent, a logic high is equivalent to a potential of about Vcc-0.8 V and a logic low is equivalent to a potential of about Vcc-1.9 V. Thus, in mating CMOS and non-CMOS transmissions, it can be seen that variations in potential swings will not automatically ensure the triggering of a desired swing from one logic level to another. Furthermore, minor potential swings in TTL signals, and low voltage TTL (LVTTL) swings in particular, may not effect any logic level change associated with CMOS transistors connected thereto. Alternatively, signal swings that do not reach full CMOS potentials, whether high or low, or that at least do so relatively slowly, can cause both pull-up (P-type MOS) transistor and pull-down (N-type MOS) transistor to be on at the same time. This produces a direct rail-to-rail current flow through the on PMOS and NMOS transistors. This current is known as simultaneous conduction current or leakage current and is an undesirable source of power consumption.
Power-on rest circuits come in a variety of designs. One circuit that has been found to be adequate in prior systems requiring the translation of a reset signal at one potential to a signal at a higher potential is illustrated in a simplified diagram in FIG. 1. The prior reset circuit with translation 10 shown includes a first reset sub-circuit 20 powered by first high-potential power rail Vcca at a first potential, and a second reset sub-circuit 30 powered by a second high-potential power rail Vccb at a second potential higher than the first potential of Vcca. A simple logic-level translator sub-circuit 40 increases the potential associated with reset sub-circuit 20 to that associated with reset sub-circuit 30 to enable appropriate interfacing of the reset sub-circuit 20 with the reset sub-circuit 30 and following circuitry coupled thereto. An exemplar suitable translator sub-circuit is shown in FIG. 2. The translated output signal at output node B is one input to any sort of logic gate capable of controlling the state of the output of circuit 10 including, for example, logic NOR gate NOR. The output of inverter IV4 is the other input to NOR. When potentials of circuits 20 and 30 have exceeded some defined value set, the outputs of 40 and IV4 are both low and the power-on reset signal is activated in that RESET will be a logic high at the potential of Vccb.
With continuing reference to FIG. 1, sub-circuit 20 includes a voltage divider formed of resistors R1 and R2, wherein R1 has a high-potential node coupled to high-potential power rail Vcca and R2 has a low-potential node coupled to a common low-potential power rail GND. Resistor R3 coupled in series with NMOS transistor Ml acts as an inverter, wherein the high-potential node of R3 is coupled to Vcca and its low-potential node is coupled to the drain of Ml. Ml has its source coupled to GND and its gate coupled to the output of the voltage divider R1/R2. As the potential of Vcca ramps up, it reaches some level defined by the voltage divider at which the output of inverter R3/M1 flips from a logic high to a logic low. That output signal is twice inverted at full Vcca potential by inverters IV1 and IV2 such that the output of IV2 at node A is the equivalent of a logic low when Vcca reaches full potential. It is to be noted that other switching means may be used as a substitute for transistor Ml, including, for example, a bipolar transistor.
Sub-circuit 30 is similarly configured. Specifically, sub-circuit 30 includes a voltage divider formed of resistors R4 and R5, wherein R4 has a high-potential node coupled to high-potential power rail Vccb at a first potential, and R5 has a low-potential node coupled to GND. Resistor R6 coupled in series with NMOS transistor M2 acts as an inverter, wherein the high-potential node of R6 is coupled to Vccb and its low-potential node is coupled to the drain of M2. M2 has its source coupled to GND and its gate coupled to the output of the voltage divider R4/R5. As the potential of Vccb ramps up, it reaches some level defined by the voltage divider at which the output of inverter R6/M2 flips from a logic high to a logic low. That output signal is twice inverted at full Vccb potential by inverters IV3 and IV4 such that the output of IV4 is the equivalent of a logic low when Vccb reaches full potential. Of course, as with the transistor M1, any sort of switching device may be used to perform the function of transistor M2 including, for example, a bipolar transistor.
The logic-level translator 40 is shown in FIG. 2. It includes inverter IV5 powered by Vcca and GND. The input to IV5 is the output of inverter IV2 from sub-circuit 20 at node A. The translator 40 further includes PMOS transistor M3, and inverters IV6 and IV7. Second high-potential rail Vccb and GND power all three devices. The output of inverter IV5 is the input to IV6 and to IV7. The output of IV6 is coupled to the gate of M3, which has its source coupled to Vccb and its drain coupled to the output of IV5. The translator 40 is designed such that inverter IV7 outputs a logic low signal at node B only when the output of IV5 is high and M3 is on. In order for IV5 to produce a logic high, Vcca must be at a potential high enough to turn on M1 of sub-circuit 20. In order for M3 of translator 40 to be on, the input to IV6 must be high and the potential of Vccb must exceed some threshold value greater than the potential associated with the logic high signal output by IV5. When that occurs, the power supply is deemed to have reached a suitable condition to trigger activation of the RESET signal for coupling to downstream circuitry.
Unfortunately, there exists a problem with the use of the translator sub-circuit 40 shown in FIG. 2 in combination with the reset circuit 10 shown in FIG. 1. Specifically, under the conditions when inverter IV5 produces a logic high signal and Vccb is essentially off and therefore establishes no potential, there is created a leakage path for current to pass from the output of IV5 through turned-on transistor M3 to Vccb. This results in the undesirable development of the leakage current that draws power unnecessarily from the system and reduces operating efficiency. More generally, it is to be noted that translation circuits use power, create signal propagation delays, may not provide full translation, and inherently take up premium circuit space.
Therefore, what is needed is a power-on reset circuit for use with a dualsupply power system wherein the two supplies may be of unequal potential. What is also needed is such a power-on reset circuit without current leakage pathways. Further, what is needed is a power-on reset circuit for the propagation of a supply signal associated with power supplies of uneven potential without requiring a translator circuit to convert logic signals of differing potentials.